Variable inductance



June 17, 1930. P. P. CIOFFI VARIABLE INDUCTANCE Filed July 8, 1924 CURVEA 65 "/0 NI 55 /o FE AT ZERO TLNSIGN CURVE. B 65% NI 5570 FE. AT 5000LBS./5Q.IN. CURVE C PURE FE AT ZERO TENlON CURVE. D PURE. FE AT 5000LBS./5Q.IN.-

+z 3 amnion a I Maqnd'izinq force Patented June 17, 1930 UNITED STATESPATENT OFFICE,

PAUL CIOFFI, OI BBOOEYN, mew YORK, ASSIGNOR TO WESTERN ELECTRIC OOH-PA'NY, OF NEW YORK, N. Y A CORPORATION OF YORK VARIABLE INDUGIANCEApplication fled July 8,

This invention relates to a variable inductance and its object is tomake possible a continuous variation in inductance over a wide range andwithout change in resistance. Such a variable inductance is ofparticular interest in balancing networks employed where it is desiredto operate duplex systems over loaded telegraph wires. It is also usefulin the shaping networks used at the terminals of long loaded cables.

In accordance with a feature of the invention, the core of an inductancecoil is composed of a material, the permeability of which is highlysensitive to mechanical strain or to change in the magnetizing force, orto both, and means are provided for varying the strain exerted upon thecore member or the strength of the magnetizing field, or both. The factthat iron and also nickel change their magnetic properties with a changein pressure or tension is well known and is commonly referred to as themagneto-stricture characteristic of the material. However, in thesematerials, this change is too small to be of any commercial advantage.In the case of certain nickel-iron alloys, when properly heat treated,the magneto-stricture characteristic is very pronounced and in a conding application of Oliver E. Buckley, rial No. 610,015, filed December30, 1922, which has issued as Patent No. 1,666,680, on April 17, 1928,this property is taken advantage of in the construction of adynamometer. As a result of an exhaustive study of the magneto-stricturecharacteristic of various nickeliron alloys, it has been determined thatthe change in permeability due to change in tension is not only greatlyvariable as regards the composition of the alloy but is also dependentto a large degree upon the heat treatment to which the composition issubjected. Thus, with a given composition, it is posible by varying theheat treatment to obtain widely difierent magneto-stricturecharacteristics. Furthermore, in the case of nickebiron alloys subjectedto mechanical strain, the rate of change in permeability with change inmagnetizing force, depends not only upon the proportion of nickel andiron' m in the composition, but also upon the heat alloy as comparedwith iron when subjected 1924. Serial No. 724,768.

tieatment to which the composition is subjecte The various features ofthe invention may be clearly understood by reference to the accompanyingdrawing in which:

Fi 1 shows a variable inductance coil embodying the features of theinvention, and

Fig. 2 shows curves illustrating the change in permeability of thepreferred nickel-iron to varying mechanical strains and to varyingmagnetizing forces.

Referring to the drawing, the core member 5 preferably consists of atape of nickel-iron alloy containing nickel and 35% iron, wound aroundthe saddle blocks 6, 6, the ends being secured by some such means as therivet 7. The lower saddle block 6 is secured by means of a ring member 8to a support 9 secured in the base 10 of the housing 11. The uppersaddle block is secured by means of ring 12 to a support member 13 whichis pivotally secured to the hinged member 14 which in turn is pivoted at15. The thumb screw 16, threaded into the free end of member 14, isadapted to bear against the cover member 17 and thus permit placing thecore member 5 under tension. About the core member 5 is placed a winding18, the ends of which are preferably brought out and connected to asignaling line over a pair of w1res such as shown at 25 and 26 byterminals not, shown.

In order further to provide for change in the inductance of the winding,a source of (11- rect current 19 connected in series with the variableresistance 20 and impedance coils 21, 22 is connected in shunt with thewinding 18, condensers 23, 24 being connected as shown to prevent directcurrent from the source '19 from flowing out on the line over conductors25 and 26. The function of the impedance coils is to prevent the directcurrent clrcuit from affecting the alternating current circuit.

The ma etic tape used for the core maybe prepared y fusing together ironand n ckel in the proportion of 65% nickel and 35% 1 ron. The moltencomposition is then poured in a mould and is rolled or otherwise ormed1nto a tape of the desired dimensions. A preferred 1 00 method ofpreparing this material is to mix together pure iron and nickel powdersin the proportions stated, compressing the mixture to form a solid massand then subjecting the composition to the temperature at which nickelfuses, the fusin step being conducted under vacuum to preclude anypossibility of contamination. To develop the utmost permeability in thistape, it is heated to a temperature of 960 C. and maintained at thistemperature for a period of approximately ten minutes. It is then cooledrapidly at the rate of approximately 30 C. per minute for a peof tenminutes and then cooled more slowly so that it attains room temperaturein approximately minutes.

Although the most satisfactory results are obtained when using an alloycontaining 65% nickel and 35% iron, very good results may also beobtained with other nickel-iron alloys in which the nickel componentpredominates and, under some conditions, with alloys in which the nickelcontent is as low as 45%. Obviously, for different alloys it isnecessary to determine experimentally the heat treatment required .todevelop the maximum permeability of the material and the magnetizingforces at which the maximum rate of change in permeability takes place.With the preferred composition ,containing 65% nickel, the permeabilitywith zero tension is relatively low and the total change in permeabilityis great. Therefore, with this material, the efiect of eddy currents isless than would be the case if the range over which the permeabilityvaries was higher.

In Fig. 2, curve A shows the change in permeability of the nickel-ironalloy containmg 65% nickel and 35% iron for various magnetizing forceswhen subjected to zero tension and curve B shows the correspondingchange in permeability when this material is I subjected to a tension of5000 pounds per square inch. In a similar manner, curve C shows thechange in permeability for various magnetizing forces of a very puregrade of iron when subjected to zero tension and curve D shows thechanges in permeability under similar conditions when this material issubjected to a tension of 5000 pounds per square inch. It will be notedthat with the nickel- 1ron alloy when employing a magnetizing force ofapproximately .17 gauss, an increase in tension from zeroto 5000 poundsper square ,inch produces an increase in permeability of fromapproximately 3000 to 51,000, that is, an increase in inductance ofapproximately sixteen times is produced by applying a tension equivalentto 5000 pounds per square inch. It will also be noted from curve B thatwith the nickel-iron composition subjected to a tension of 5000 poundsper square inch, a change in the magnetizing force of from zero to .17gauss produces a change in permeability of from approximately 6500 to Inthe arrangement disclosed in F1g. 1, a strip .006 of an inch thick and.25 of an mch wide, composed of a nickel-iron alloy containimatcly eightinches. When provided with a proper winding to secure the desiredmagnetizing force of .17 gauss, the inductance for zero tension wasequal to a proximately 1 henry and when the core mem r was sub ected toa tension of 7 pounds, the inductance was increased to approximately 17henr s. By means of the leverage of the hinge 14,t 1s change ininductance necessitated a pressure of only 3% pounds in line -with thethumb screw 16.

The shape of curve B of Fi 2 may varied by introducing air ga s odesired w dth at the top or bottom or bot of the nlckel-lron alloy strip5. In general, this will lower the permeability and make the device lesssensitive to small variations of current from battery 19.

- What is claimed is:

1. In a variable inductance arrangement, a core of an iron-nickel alloyhaving a permeability which may be varied over a wide range undermechanical strain, meohamcal means for applying strain to said core toincrease its permeability range, a fixed winding about said core, aseries path including saidwinding and also including a pair of wireshaving terminals adapted for connection to a circuit, an adjustabledlrect current source connected to said path for producing abiasingcurrent in said winding tqmalntain a high average permeability of saidcore, a stopping condenser in said path separate said source from saidpair of wires and a choke coil connected between said source and saidpair of wires.

2. In an inductance control arrangement, a core and a winding thereon,said winding having an inductance which is controllable by variation ofmechanical strain apphed to said core to compensate for variations in.of permeability variation is obtained in response to variations withinsaid limits of the strain, inductive means included in such directcurrent circuit for choking alternating currents therein, and analternating current circuit for superimposing an alternating flux on thebiasing flux resulting from said magnetomotive force of a magnitudedetermined by the adjustment of said mechanical means, said alternatingcurrent circuit including a capacitance for obstructing the flow ofdirect current therein.

3. In an electric circuit including an inductive device having a corewith a permeability which is highly sensitive to mechanical strain, themethod of varying the inductance of said circuit which comprisesproducing a unidirectional biasing flux by applying a magnetizing forceto said core to secure high sensitivity of its permeability at said fluxto variations in strain and applying a mechanical strain to said core tosecure a desired permeability and adjusting said strain to fiect adiflerent ermeabi ity of said flux.

4. A method 0 operating a variable inductance having a core of magneticmaterial the permeability of which, at a given magnetizing force, variesover a wide range in accordance with variations in mechanical strainapplied to said material which comprises. applying a unidirectionalmagnetizing force to said core to secure said wide range ofpermeabilities and varying the mechanical strain in said material toproduce any permeability, due to said magnetizin force, within saldrange, and applying an a tuna]:- ing magnetizin force to said core topr0- duce an alternating flux of a magnitude varyin with the mechanicalstrain.

witness where0f,I hereunto subscribe my name this 7th day of July, A. D.1924. PAUL P. GIOFFI.

